Much recent research has been devoted to developing improvements in the detection of invisible light such as infrared light, ultraviolet light, or X-rays. Such research is expected to have many applications.
By way of example, detection of infrared radiation has important applications. Infrared light is a form of energy that radiates from all objects having a non-zero temperature (i.e., a temperature above zero degrees K). For example, an object having a temperature of approximately 300K gives off infrared radiation having a peak in the range of 8 to 12 .mu.m. Such radiation can be used to detect the presence, shape, and temperature of an object, even in complete darkness. For example, infrared radiation can be used to guide a motor vehicle at night with no illumination as easily as if the vehicle were being driven in the daytime. Infrared light also can be used to detect the presence of a person illegally trespassing in a building without illumination at night.
In view of the many applications of detecting infrared light, various approaches have been developed for detecting such light since the discovery of infrared light by Herschel in about 1800. Currently, infrared detectors generally are classified into two types of detectors: quantum type infrared detectors and thermal type infrared detectors.
Despite the foregoing advances, detection of infrared light is still technically difficult, thus limiting the current scope of infrared detection technology.
A quantum type infrared detector converts photon energy (E=h.nu.) of infrared light to electronic energy which is then detected, typically using charge-coupled devices. As indicated above, the infrared light most useful for technological applications has a wavelength range of 3 to 12 .mu.m. Photons of such wavelength have an energy of about 0.1 to 0.4 eV, which is approximately equal to the heat energy of an electron at room temperature.
In order to convert the photon energy of incident infrared radiation to electronic energy, the influences of any thermal energy of the electrons must be eliminated (especially since the photon energy of an infrared photon is very small and nearly equal to the thermal energy of an electron at room temperature). This requires that a quantum-type infrared detector be cooled. It is normally necessary to cool the detector to about -200.degree. C. (77K). Unfortunately, achieving such a level of cooling requires an auxiliary cooling device that occupies excessive volume, has excessive mass, generates excessive mechanical vibrations, has a short operational lifetime, and has excessively high cost. Thus, it is difficult to reduce the size and cost of an infrared camera or the like that comprises a conventional quantum-type infrared detector, which prohibits widespread use of such infrared cameras.
A conventional thermal-type infrared detector converts the energy of incident infrared radiation to thermal energy. The added thermal energy causes localized temperatures of the detector to change. Such localized temperature changes cause (corresponding localized changes in a measurable parameter of the detector, such as the electrical resistance of a bolometer at the particular location experiencing a temperature change.
Although a conventional thermal-type infrared detector normally does not require a cooler, in contrast to the conventional quantum-type infrared detector, a conventional thermal-type infrared detector exhibits certain problems. For example, in order to detect a localized temperature change, it is necessary to have an electric current flow through the detector at that location. Such current flow causes localized self-heating which makes it difficult to detect a localized temperature change caused solely by the incident infrared radiation. As a result, detection accuracy is reduced.
Further with respect to conventional thermal-type infrared detectors, each location of the detector at which a localized temperature change is to be measured normally is suspended in air above the substrate in such a way that physical contact with the substrate is minimal. This provides sufficient thermal isolation of each location sufficiently to allow the localized temperature change caused by absorbed incident infrared radiation at that location to be detected.
It usually is necessary to provide each such suspended location with its own respective bolometer or the like, which complicates the making of electrical connections to the bolometer. This problem usually is solved by incorporating electrically conductive material in the fabrication of each bolometer. Unfortunately, electrically conductive materials tend to have extremely high thermal conductivities. This makes it difficult to improve the thermal insulation of each of the suspended portions relative to the substrate in a conventional thermal-type infrared detector. Inadequate thermal insulation contributes to a degradation of detection accuracy and detector sensitivity.
Conventional thermal-type infrared detectors also disadvantageously exhibit low sensitivity. For example, the bolometers in a conventional thermal-type infrared detector typically comprise a material that exhibits a resistance change of abort 2 percent for every 1.degree. C. change in temperature of the material. The efficiency with which infrared radiation is converted to a measurable resistance change is at most about 1 percent. Hence, with every 1.degree. C. change in temperature of an object to be measured, the resistance of the bolometers of the detector changes by 0.02 percent.
As a result, the electrical signal produced by a conventional thermal-type infrared detector is very weak. This requires that the electronic circuit used for reading out the electric signal he able to greatly reduce background noise with high amplification.
A conventional thermal-type infrared sensor also disadvantageously is affected easily by changes in external temperature. Thus, the output of the sensor easily fluctuates. If the radiation-to-heat conversion efficiency of such sensors is at most 1 percent, then temperature changes of the sensor must be less than 0.001.degree. C. to ensure a temperature-detection precision of 0.1.degree. C. Thus, very strict temperature control of the sensor is required.
The problems with conventional infrared detectors summarized above are generally applicable to conventional detectors for other types of electromagnetic radiation such as detectors for ultraviolet light and X-rays.